Mayank Uttam

CBSE board exams 2018 are about to start from 5 March. Now, few days are left for the final preparation and this is the time when selective study plays a crucial role in the final preparation. If we carefully observe previous years’ papers of Class 12 Science (Physics, Chemistry, Biology) and Maths board exams, then we can easily understand that questions based on some concepts are frequently asked in board exams, every year.

In order to help the students, we have provided 120 important questions for Class 12 Maths, Physics, Chemistry, and Biology (30 questions for each subject). These questions are mostly based on important concepts from which questions are expected every year.

In order to score well in CBSE board exam 2018 result, Class 12th students having Physics, Chemistry, Biology and Maths as one of their subjects must practice these questions with pen and paper.

Students can find solutions to the questions from various solved guess papers, practice papers and sample papers provided by Jagranjosh.com. Most of the papers are free of cost (except Guess Papers).

Important questions for Class 12 Physics board exam 2018

Question 1:

(a) Draw a ray diagram of an astronomical telescope for the final image formed at least distance of distinct vision?

(b) An astronomical telescope has an angular magnification of magnitude 5 for distant objects. The separation between the objective and an eye piece is 36 cm and the final image is formed at infinity. Calculate the focal length of the objective and the focal length of the eye piece?

Question 2:

State the factors on which the refractive index of a material medium for a given wavelength depends.

Question 3:

A metallic rod of length l is rotated at an angular speed w, normal to a uniform magnetic field B. Derive an expression for the (i) emf induced in the rod (ii) heat dissipation, if the resistance of the rod is R.

Question 4:

(a) Explain the meaning of the term mutual inductance. Consider two concentric circular coils, one of radius r1 and the other of radius r2(r1<r2) placed coaxially with centres coinciding with each other. Obtain the expression for mutual inductance of the arrangement.

(b)A rectangular coil of area A, having number of turns N is rotated at ‘f’ revolution per second in a uniform magnetic field B, the field being perpendicular to the coil. Prove that the maximum emf induced in the coil is 2πf NBA.

With a circuit diagram, explain how a Zener diode can be used as a voltage regulator.

Question 6:

If N drops of same size each having the same charge, coalesce to form a bigger drop. What is change in the charge, capacity and potential of the bigger drop with respect to smaller drop?

Question 7:

What do you understand by root mean square value of alternating current? Derive an expression to show that the root mean square value of alternating current is 0.707 times the value of alternating current.

Question 8:

Write two basic modes of communication. Explain the process of amplitude modulation. Draw a schematic sketch showing how amplitude modulated signal is obtained by superposing a modulating signal over a sinusoidal carrier wave.

Question 9:

What is meant by ‘half-life’ of a radioactive element? What percentage of a given mass of a radioactive substance will be left undecayed after four half lives?

Question 10:

A number of identical cells, n, each of emf E, internal resistance r connected in series are charged by a d.c. source of emf E’, using a resistor R.

(i) Draw the circuit arrangement.

(ii) Deduce the expressions for (a) the charging current and (b) the potential difference across the combination of the cells.

Question 11:

Define resistivity of a conductor. Plot a graph showing the variation of resistivity with temperature for a metallic conductor. How does one explain such a behaviour, using the mathematical expression of the resistivity of a material.

Question 12:

A parallel plate capacitor, each with plate area A and separation d, is charged to a potential difference V. The battery used to charge it remains connected. A dielectric slab of thickness d and dielectric constant k is now placed between the plates. What change, if any, will take place in:

(i) charge on plates?

(ii) electric field intensity between the plates?

(iii) capacitance of the capacitor?

Question 13:

State the working principle of an AC generator with the help of a labelled diagram. Derive an expression for the instantaneous value of the emf induced in coil. Why is the emf maximum when the plane of the armature is parallel to the magetic field?

Question 14:

In a single slit diffraction experiment, when a tiny circular obstacle is placed in the path of light from a distant source, a bright spot is seen at the centre of the shadow of the obstacle. Explain why?

Question 15:

(a) Define self inductance. Write its S.I. units.

(b) Derive an expression for self inductance of a long solenoid of length l, cross-sectional area A having N number of turns.

Question 16:

(i) Can two equi-potential surfaces intersect each other? Give reasons.

(ii) Two charges -q and + q are located at points A (0, 0, - a) and B (0, 0, + a) respectively.

How much work is done in moving a test charge from point P (7, 0, 0) to Q (-3, 0, 0)?

Question 17:

Using the postulates of Bohr's model of hydrogen atom, obtain an expression for the frequency of radiation emitted when atom make a transition from the higher energy state with quantum number ni to the lower energy state with quantum number nf (nf < ni).

Question 18:

A parallel plate capacitor is charged by a battery. After sometime the battery is disconnected and a dielectric slab with its thickness equal to the plate separation is inserted between the plates. How will (i) the capacitance of the capacitor, (ii) potential difference between the plates and (iii) the energy stored in the capacitor be affected?

Justify your answer in each case.

Question 19:

Write Einstein’s photoelectric equation. State clearly how this equation is obtained using the photon picture of electromagnetic radiation.

Write the three salient features observed in photoelectric effect which can be explained using this equation.

Question 20:

(a) The bluish colour predominates in clear sky.

(b) Violet colour is seen at the bottom of the spectrum when white light is dispersed by a prism. State reason to explain these observations.

Question 21:

Arrange the following electromagnetic radiations in ascending order of their frequencies:

(i) Microwave

(ii) Radio wave

(iii) X-rays

(iv) Gamma rays

Question 22:

(i) A plane wavefront approaches a plane surface separating two media. If medium 'one’ is optically denser and medium 'two’ is optically rarer, using Huygens’ principle, explain and show how a refracted wavefront is constructed.

(ii) Hence verify Snell's law.

(iii)When a light wave travels from rarer to denser medium, the speed decreases. Does it imply reduction its energy? Explain.

Question 23:

Derive an expression for the de-Broglie wavelength associated with an electron accelerated through a potential V. Draw a schematic diagram of a localised-wave describing the wave nature of the moving electron.

Question 24:

Draw a plot showing the variation of binding energy per nucleon versus the mass number A. Explain with the help of this plot the release of energy in the processes of nuclear fission and fusion.

Question 25:

(a) What are eddy currents? How are these currents reduced in the metallic cores of transformers?

(b) A step down transformer operates on a 2.5 KV line. It supplies a load with 20 A. The ratio of the primary winding to the secondary is 10: 1. If the transformer is 90% efficient, calculate:

(i) the power output,

(ii) the voltage, and

(iii) the current in the secondary

Question 26:

Use Huygen’s principle to explain the formation of diffraction pattern due to a single slit illuminated by a monochromatic source of light. When the width of slit is made double the original width, how this affect the size and intensity of the central diffraction band?

Question 27:

Name the type of waves which are used for line of sight (LOS) communication. What is the range of their frequencies?

A transmitting antenna at the top of a tower has a height of 45 m and the receiving antenna is on the ground. Calculate the maximum distance between them for satisfactory communication in LOS mode. (Radius of the Earth = 6.4 × 106 m).

Question 28:

Draw the typical input and output characteristics and explain how these graphs are used to calculate current amplification factor of the transistor.

Question 29:

Using Gauss’s law obtain the expression for the electric field due to a uniformly charged thin spherical shell of radius R at a point outside the shell. Draw a graph showing the variation of electric field with r, for r > R and r < R.

Question 30:

Draw a circuit diagram to study the input and output characteristics of an n-p-n transistor in its common emitter configuration.

Important questions for Class 12 Chemistry board exam 2018

Question 1:

Explain the following properties giving suitable examples:

(i) Ferromagnetism

(ii) Paramagnetism

(iii) Ferrimagnetism

Question 2:

Write the names and structure of the monomers of the following polymers:

What is meant by the ‘rate constant, k’ of a reaction? If the concentration is expressed in mol L-1 units and time in seconds, what would be the units for k (i) for a zero order reaction and (ii) for a first order reaction?

Question 5:

Calculate the temperature at which a solution containing 54 g of glucose, (C6H12O6), in 250 g of water will freeze. (Kf for water =1.86 K mol‒1 kg)

Question 6:

Assign reasons for the following:

(i) The enthalpies of atomisation of transition elements are high.

(ii) The transition metals and many of their compounds act as good catalysts.

(iii) From element to element the actinoid contraction is greater than the lanthanoid contraction.

(iv) The E° value for the Mn3+ / Mn2+ couple is much more positive than that of Cr3+ / Cr 2+.

(v) Scandium (Z = 21) does not exhibit variable oxidation states and yet it is regarded as a transition element.

Question 7:

What happens when D-glucose is treated with the following reagents?

(i) HNO3

(ii) Bromine water

(iii) HI

Indicate the products formed.

Question 8:

Account for the following:

(i) pKbof methylamine is less than that of aniline.

(ii) Aniline does not undergo Friedel–Crafts reaction.

(iii) Ethylamine is freely soluble in water whereas aniline is only slightly soluble.

Question 9:

Compare actinoids and lanthanoids with reference to their:

(i) electronic configurations of atoms

(ii) oxidation states of elements

(iii) general chemical reactivity of elements.

Question 10:

(a) Depict the galvanic cell in which the following reaction takes place:

Zn (s) + 2Ag+ (aq) → Zn 2+ (aq) + 2Ag (s)

Also indicate that in this cell

(i) Which electrode is negatively charged.

(ii) What are the carrier of the current in the cell.

(iii) What is the individual reaction at each electrode.

Question 11:

Define the following in relation to proteins:

(i) Primary structure

(ii) Denaturation

(iii) Peptide linkage

Question 12:

(a) Complete the following chemical reaction equations:

(i) HgCl2 (aq) + PH3 (g) →

(ii) SiO2 (s) + HF (g) →

(b) Explain the following observations:

(i) Sulphur in vapour state exhibits paramagnetic behaviour.

(ii) The stability of +3 state increases down the group in group 15 of the periodic table.

(iii) XeF2 has a linear shape and not a bent structure.

Question 13:

What happens in the following activities and why?

(i) An electrolyte is added to a hydrated ferric oxide sol in water.

(ii) A beam of light is passed through a colloidal solution.

(iii) An electric current is passed through a colloidal solution.

Question 14:

Corrosion is essentially an electrochemical phenomenon. With the help of a diagram explain the reactions occurring during the corrosion of iron kept in open atmosphere.

Question 15:

Explain why

(i) H2O is a liquid while, inspite of a higher molecular mass, H2S is a gas.

(ii) Iron dissolves in HCl to form FeCl2 and not FeCl3.

(iii) Helium is used in diving equipment.

Question 16:

Explain the role of each of the following in the extraction of metals from their ores:

(i) CO in the extraction of nickel.

(ii) Zinc in the extraction of silver.

(iii) Silica in the extraction of copper.

Question 17:

State the following:

(i) Henry’s law about partial pressure of a gas in a mixture.

(ii) Raoult’s law in its general form in reference to solutions.

Question 18:

State Kohlrausch law of independent migration of ions. Write an expression for the molar conductivity of acetic acid at infinite dilution according to Kohlrausch law.

Question 19:

Express the relation among the cell constant, the resistance of the solution in the cell and the conductivity of the solution. How is the conductivity of a solution related to its molar conductivity?

Question 20:

Explain the following cases giving appropriate reasons:

(i) Nickel does not form low spin octahedral complexes.

(ii) The π-complexes are known for the transition metals only.

(iii) Co2+ is easily oxidised to Co3+ in the presence of a strong ligand.

Question 21:

Explain the following terms with an example for each:

(i) Antibiotics

(ii) Antiseptics

(iii) Analgesics

Question 22:

Account for the following:

(i) The acidic strength decreases in the order HCl > H2S > PH3.

(ii) Tendency to form pentahalides decreases down the group in group 15 of the periodic table.

Question 23:

For the complex [Fe (en)2 Cl2] Cl, identify the following:

(i) Oxidation number of iron.

(ii) Hybrid orbitals and shape of the complex.

(iii) Magnetic behaviour of the complex.

(iv) Number of its geometrical isomers.

(v) Whether there may be optical isomer also.

(vi) Name of the complex.

Question 24:

How would you account for the following:

(i) NF3 is an exothermic compounds but NCl3 is not.

(ii) The acidic strength of compounds increases in the order:

PH3 < H2S < HCl

(iii) SF6 is kinetically inert.

Question 25:

Calculate the amount of KCl which must be added to 1 kg of water so that the freezing point is depressed by 2 K. (Kffor water = 1.86 K kg mol–1)

Question 26:

What are ambident nucleophiles? Explain giving an example.

Question 27:

Differentiate among a homogeneous solution, a suspension and a colloidal solution, giving a suitable example of each.

Question 28:

Draw the structures of white phosphorus and red phosphorus. Which one of these two types of phosphorus is more reactive and why?

Question 29:

What is the difference between multimolecular and macromolecular colloids? Give one example of each type. How are associated colloids different from these two types of colloids?

Question 30:

(i) State one use each of DDT and iodoform.

(ii) Which compound in the following couples will react faster in SN2 displacement and why?

(a) 1-Bromopentane or 2-bromopentane

(b) 1-Bromo-2-methylbutane or 2-bromo-2-methylbutane.

Important questions for Class 12 Biology board exam 2018

Question 1:

(a) State how the constant internal/environment is beneficial to organisms.

(b) Explain any two alternatives by which organisms can overcome stressful external conditions.

Question 2: Describe the Hershey and Chase experiment. Write the conclusion drawn by the scientists after their experiment.

Question 3: With advancements in genetics, molecular biology and tissue culture, new traits have been incorporated into crop plants. Explain the main steps in breeding a new genetic variety of a crop.

Question 4: Since the origin of life on Earth, there were five episodes of mass extinction of species.

(i) How is the ‘Sixth Extinction’, presently in progress, different from the previous episodes?

(ii) Who is mainly responsible for the ‘Sixth Extinction’?

(iii) List any four points that can help to overcome this disaster.

Question 5:

(a) Write the blood group of people with genotype IAIB. Give reasons in support of your answer.

(b) In one family, the four children each have a different blood group. Their mother has blood group A and their father has blood group B. Work out a cross to explain how it is possible.

Question 6:

(a) Explain “founder effect”.

(b) State Oparin and Haldane Hypothesis.

(c) Describe Stanley and Miller’s experiment and give its significance.

Question 7:

Explain the role of the following in increasing the soil fertility and crop yield:

(a) Leguminous plants

(b) Cyanobacteria

(c) Mycorrhizae

Question 8:

Explain the role of the following in increasing the soil fertility and crop yield:

(a) Leguminous plants

(b) Cyanobacteria

(c) Mycorrhizae

Question 9:

A burglar in a huff forgot to wipe off his blood-stains from the place of crime where he was involved in a theft and fight. Name the technique which can help in identifying the burglar from the blood-stains. Describe the technique.

Question 10:

(a) Explain DNA polymorphism as the basis of genetic mapping of human genome.

(b) State the role of VNTR in DNA fingerprinting.

Question 11:

Explain the role of the following in increasing the soil fertility and crop yield:

(a) Leguminous plants

(b) Cyanobacteria

(c) Mycorrhizae

Question 12: (a) List the biotic components an organism interacts with in its natural habitat.

(b) Mention how have organisms optimised their survival and reproduction in a habitat.

Question 13:

Identify A, B, C and D in the table given below.

Crop

Variety

Resistance to disease

Wheat

A

Leaf and stripe rust

B

Pusa Shubhra

Black-rot

Cowpea

Pusa Komal

C

Brassica

Karan Rai

D

Question 14:

Draw a transverse sectional view of an apple and label the following parts along with their technical names:

(i) edible part

(ii) encloses the embryo

(iii) forms the fruit wall

Question 15:

(a) Draw a sectional view of human ovary and label

(i) Primary follicle

(ii) Graafian follicle

(iii) Corpus luteum

(b) Mention the effect of pituitary hormones on the parts labelled.

Question 16:

(a) Name the stage of Plasmodium that gains entry into the human body.

(b) Trace the stages of Plasmodium in the body of female Anopheles after its entry.

(c) Explain the cause of periodic recurrence of chill and high fever during malarial attack in humans.

Question 17:

Define totipotency of a cell. List the requirements if the objective is to produce somaclones of a tomato plant on commercial scale.

Question 18:

(a) Expand BOD.

(b) At a particular segment of a river near a sugar factory, the BOD is much higher than the normal level. What is it indicative of? What will happen to the living organisms in this part of the river?

(c) Under what conditions will the BOD be lowered in the river? How will it affect the aquatic life?

Explain with the help of two examples how certain plants have evolved morphological and chemical defenses against primary consumers such as cows and goats.

Question 21:

(a) State one difference and one similarity between geitonogamy and xenogamy.

(b) Explain any three devices developed in flowering plants to discourage self pollination and encourage cross pollination.

Question 22:

(a) Comment on the pattern in which all communities undergo a change in composition and structure with changing environmental conditions.

(b) Explain ‘Climax community’ and ‘sere’.

(c) Differentiate between primary and secondary succession with examples.

Question 23:

Human female is not fertile after menopause whereas males can produce gametes at any age after puberty. Analyse the statement and schematically represent a comparison between gametogenesis in males and females.

Question 24:

What is the pathogenic property of baculovirus, used as a biological agents ? Name the genus of these organisms.

What happens when chromatids fail to segregate during cell division cycle? Explain your answer with an example.

Question 28:

Excessive and continuous use of pesticides has resulted in evolution of some new species of pests. Explain what must have led to this. What is this type of evolution called?

Question 29:

Describe the various steps of Griffith’s experiment that led to the conclusion of the ‘transforming principle’.

Question 30:

What type of organs eye of an Octopus and that of a human called? Give another example from the animal group and one from the plants of such organs. Name and explain the evolutionary process they exhibit.

(i) Is the binary operation defined on set N, given by a * b = (a + b)/2 for all a, b ϵ N, commutative?

(ii) Is the above binary operation associative?

Question 10:

Find the distance between the lines l1 and l2 given by

Question 11:

An insurance company insured 2000 scooter drivers, 4000 car drivers and 6000 truck drivers. The probabilities of an accident for them are 0.01, 0.03 and 0.15 respectively. One of the insured persons meets with an accident. What is the probability that he is a scooter driver or a car driver?

Question 12:

If a 20 year old girl drives her car at 25 km/h, she has to spend Rs 4/km on petrol. If she drives her car at 40 km/h, the petrol cost increases to Rs 5/km. She has Rs 200 to spend on petrol and wishes to find the maximum distance she can travel within one hour. Express the above problem as a Linear Programming Problem. Write any one value reflected in the problem

Question 13:

Find the shortest distance between the line x ‒ y + 1 = 0 and the curve y2 = x.

Prove that the relation R on the set A = {1, 2, 3, 4, 5} given by R = {(a, b):|a ‒ b| is even }, is an equivalence relation

Question 22:

Question 23:

Question 24:

Question 25:

The probabilities of two students A and B coming to the school in time are 3/7 & 5/7 respectively. Assuming that the events, ‘A coming in time’ and ‘B coming in time’ are independent, find the probability of only one of them coming to the school in time. Write at least one advantage of coming to school in time.

Question 26:

In a hockey match, both teams A and B scored same number of goals up to the end of the game, so to decide the winner, the referee asked both the captains to throw a die alternately and decided that the team, whose captain gets a six first, will be declared the winner. If the captain of team A was asked to start, find their respective probabilities of winning the match and state whether the decision of the referee was fair or not.

Question 27: For what value of k is the following function continuous at x = 2?